For industrial applications, multi-stage steam turbines are often designed to extract working fluid, e.g., steam, at definite pressures from one or more intermediate stages of the multi-stage steam turbine as the steam travels from an inlet to an outlet of the multi-stage steam turbine. As the steam travels from the inlet to the outlet, the pressure of the steam may drop (e.g., successively in each stage) while the steam gradually expands. Thus, steam extracted from an intermediate stage may have a pressure less than the pressure of the steam at the inlet and greater than the pressure of the steam at the outlet.
One way of extracting steam of a desired pressure is to let the steam “bleed” out from an intermediate stage via an opening. While this is a relatively simple way of obtaining steam of a desired pressure, the pressure of the extracted steam may not be stable, e.g., may vary based on load conditions of the steam turbine, and thus steam having a relatively fixed pressure may not be obtained.
To overcome the above drawback, the multi-stage steam turbine may include a grid valve between intermediate stages. Herein, to obtain steam having a relatively fixed pressure, the grid valve of an intermediate stage having steam of the desired pressure may be closed to hold the steam therein and prevent the steam from passing to the downstream stages. The desired fixed pressure steam may then be extracted from the intermediate stage.
FIG. 1 illustrates a cross-sectional overview of a conventional multi-stage steam turbine 100 (only 2 stages shown) including a conventional annular grid valve 102. The conventional multi-stage steam turbine 100 may include a casing 104 having a rotatable shaft 106 mounted therein on suitable bearings 108 (only one of which is shown). Suitable seals 110 and 112, e.g., labyrinth seals, may be provided around the rotatable shaft 106. High pressure steam may be admitted into the casing 104 through an inlet valve 114.
The steam may flow into a relatively high pressure stage 116, wherein the steam may have a high pressure. The high pressure stage 116 may be separated from a relatively low pressure stage 118 (wherein the steam may have a low pressure relative to the high pressure stage 116) by the conventional annular grid valve 102 disposed about the rotatable shaft 106.
Typically, the conventional annular grid valve 102 may include an annular stationary plate 126 and an annular rotatable plate 128 disposed on the annular stationary plate 126 and rotatable relative to the annular stationary plate 126. The annular stationary plate 126 may define a plurality of stationary plate holes 127 circumferentially disposed therein. Likewise, the annular rotatable plate 128 may also define a plurality of rotatable plate holes 129 circumferentially disposed therein.
The annular grid valve 102 may be “opened” by rotating the annular rotatable plate 128 such that the stationary plate holes 127 and the rotatable plate holes 129 overlap each other to create a passageway for steam to pass through the annular grid valve 102 from the upstream high pressure stage 116 to the downstream low pressure stage 118. Similarly, the annular grid valve 102 may be “closed” by rotating the annular rotatable plate 128 such that the stationary plate holes 127 and the rotatable plate holes 129 do not overlap one another. When closed, steam may be prevented from passing to the downstream low pressure stage 118 and at least a portion of the steam may be extracted from the high pressure stage 116 via an extraction conduit 120.
The steam traversing the annular grid valve 102 may pass through an annular nozzle plate 124 that may be disposed on and in contact with the annular stationary plate 126 in the low pressure stage 118. The annular nozzle plate 124 may define a plurality of evenly spaced, circumferentially disposed nozzles 130. The nozzles 130 may be arranged in groups and it may be desired to provide steam to the low pressure stage 118 sequentially (one group after the other) via the groups of nozzles 130.
However, in the conventional annular grid valve 102, the stationary plate holes 127 and the rotatable plate holes 129 may overlap with each other at the same time and by the same amount (e.g., size of the passageway created due to the overlap is the same). Also, all stationary plate holes 127 may be closed the same time. Because all stationary plate holes 127 and all rotatable plate holes 129 may overlap at the same time and by the same amount, steam exiting the high pressure stage 116 may pass through all the nozzles 130 at the same time and it may thus not be possible to provide steam to the low pressure stage 118 sequentially via the nozzles 130. Also, steam entering the low pressure stage 118 in such a manner may have a throttling effect on the low pressure stage 118 which may lead to decreased efficiency of the conventional multi-stage steam turbine 100.
Additionally, the steam acting on the annular rotatable plate 128 may create a force on the respective annular mating surfaces (not shown) of the annular stationary plate 126 and the annular rotatable plate 128. This force may be a function of the differential pressure across the annular grid valve 102 and the area of the annular surface of the annular rotatable plate 128 exposed to the steam. As the differential pressure and the surface area of the annular rotatable plate 128 exposed to the steam increase, the force on the annular mating surfaces may increase and an increased actuating force may be utilized to rotate the annular rotatable plate 128. In order to provide an increased actuating force, an actuating mechanism(s) having an increased output, for example, providing an increased mechanical force, may be utilized.
What is needed, then, is a grid valve that may provide steam to a downstream stage sequentially via the nozzles, thereby reducing the throttling effect, and which may be actuated with a reduced actuating force under an increased differential pressure and exposed plate area.